Typestate

program.cs

namespace ExplicitTypes;

public interface IElementType { }

public interface IPokemon { int Id { get; } string Name { get; } }

public sealed class Fire : IElementType { }
public sealed class Water : IElementType { }
public sealed class Electric : IElementType { }
public sealed class Flying : IElementType { }
public sealed class None : IElementType { }

public sealed record Pokemon<T1, T2>(int Id, string Name) : IPokemon
    where T1 : IElementType
    where T2 : IElementType;

public static class Dex
{
    public static Pokemon<Fire, None> Charmander() => new(4, "charmander");
    public static Pokemon<Water, None> Squirtle() => new(7, "squirtle");
    public static Pokemon<Electric, Water> Lanturn() => new(171, "lanturn");
    public static Pokemon<Water, Flying> Gyarados() => new(130, "gyarados");
}

public sealed class WaterMember
{
    public readonly IPokemon _pokemon;

    private WaterMember(IPokemon pokemon) => _pokemon = pokemon;

    public static WaterMember From<T2>(Pokemon<Water, T2> p)
        where T2 : IElementType => new(p);

    public static WaterMember From<T1>(Pokemon<T1, Water> p)
        where T1 : IElementType => new(p);
}

public sealed class WaterTeam
{
    public readonly List<WaterMember> _members = new();
    public void Add(WaterMember m) => _members.Add(m);
}

public class Program
{
    public static void Main()
    {
        var team = new WaterTeam();
        team.Add(WaterMember.From(Dex.Lanturn())); // ✅
        team.Add(WaterMember.From(Dex.Gyarados())); // ✅
        team.Add(WaterMember.From(Dex.Charmander())); // ❌
    }
}